Current approaches to treating disease by administering therapeutic proteins include in vitro production of therapeutic proteins for conventional pharmaceutical delivery (e.g. intravenous, subcutaneous, or intramuscular injection) and, more recently, gene therapy.
Proteins of therapeutic interest are generally produced by introducing exogenous DNA encoding the protein of therapeutic interest into appropriate cells. For example, exogenous DNA encoding a desired therapeutic protein is introduced into cells, such as immortalized cells in a vector, such as a plasmid, from which the encoded protein is expressed. Further, it has been suggested that endogenous cellular genes and their expression may be modified by gene targeting. See for example, U.S. Pat. No. 5,272,071, WO 91/06666, WO 91/06667 and WO 90/11354.
Presently-available approaches to gene therapy make use of infectious vectors, such as retroviral vectors, which include the genetic material to be expressed. Such approaches have limitations, such as the potential of generating replication-competent virus during vector production; recombination between the therapeutic virus and endogenous retroviral genomes, potentially generating infectious agents with novel cell specificities, host ranges, or increased virulence and cytotoxicity; independent integration into large numbers of cells, increasing the risk of a tumorigenic insertional event; limited cloning capacity in the retrovirus (which restricts therapeutic applicability) and short-lived in vivo expression of the product of interest. A better approach to providing gene products, particularly one which avoids the limitations and risks associated with presently available methods, would be valuable.